The invention relates to a gear-changing apparatus of, for example, the type which connects the entire output shaft of an automobile engine to the drive axle of the automobile.
It is known (for example from the publication "Automobiltechnische Zeitschrift" 1971, pp. 161-169) to provide a gear-changing apparatus in which the arrangement which controls the operation of the gear-changing valves operates in a purely hydraulic manner. In other words, the information necessary to automatically initiate a gear change in the engine transmission is provided exclusively in the form of hydraulic pressure signals applied to the control arrangement which operates the gear-changing valves. This information relates to such engine operating variables as load (as indicated for example by the position of the gas pedal), vehicle speed (as indicated for example by the rotational speed of the transmission output shaft), and the like.
With the known arrangements, the actuation of the gear-changing valves which control the flow of fluid to the gear-changing units (such as the conventional brake bands or multiple-disk brakes and clutches) is likewise performed under the action of purely hydraulic control pressures.
When up-shifting (switching from a higher speed-reducing transmission gear ratio to a lower speed-reducing transmission gear ratio), it is desirable to avoid a transient discontinuity in the drive train. In other words, since up-shifting will generally result in a decrease of the engine output shaft rotational speed, it is important during up-shifting to prevent a transient removal of the load torque applied to the engine output shaft, since such transient removal of load torque would result in a transient engine output shaft speed increase--just the opposite of what is desired when up-shifting. To avoid such a transient discontinuity in the drive train, resort is had to a so-called "gear-shift overlap" expedient. For example, if in the lower gear a particular hydraulically actuated gear-changing member (such as a brake or clutch) is in engaged condition, and if in the next-higher gear such member is disengaged and a different gear-changing member (brake or clutch) is engaged, then the "gear-shift overlap" is caused to occur in the sense that the first such gear-changing member is not disengaged until after the second such gear-changing member has become engaged. The expression "gear-shift overlap" is appropriate because, for a brief period of time during the up-shifting operation, gear-changing members (brake bands or clutches) associated with two different tramsmission ratios are both engaged.
However, a difficulty is posed by such "gear-shift overlap" when the gear-changing members (such as brake bands) operate by the force of friction, because if two such brake bands, for example, are permitted to be engaged in this manner for a substantial period of time, then there may occur an excessive development of heat. In other words, care must be taken to assure that the time duration of the "gear-shift overlap" is optimally selected.
Such optimal selecting of the time duration of the "gear-shift overlap" can be effected in the known purely hydraulically operating transmission-controller only by means of hydraulically operating control elements. However, such hydraulically operating control elements exhibit disadvantages. Firstly, one is limited to the specific geometry and dimensions of the particular hydraulic control elements of the particular transmission-controlling hydraulic system, so that after the control system has been built and installed, subsequent changes in the duration of the "gear-shift overlap" can be made only with considerable difficulty, if at all. Secondly, the marked temperature-dependence of the viscosity of the hydraulic oil serving as the energy-transmitting medium for such hydraulic control elements can result in marked and unpredictable changes in the time duration of the "gear-shift overlap."
The same applies to down-shifting operations with such known gear-changing apparatuses (changing from a lower speed-reduction transmission ratio to a higher ratio). For down-shifting with the known apparatus, care is taken to assure that the previously engaged gear-shifting member (e.g., brake band or clutch) is disengaged at the same moment that the engagement of the gear-shifting member associated with the next transmission ratio is commanded. The practical significance of this is that, during such down-shifting operation, for a brief time neither of such two brake bands or clutches will be engaged, since there is always a time lag of a few tenths of a second between the generation of the command signal which commands engagement of such a gear-shifting member and completion of the movement of the member into fully engaged position. The resulting transient discontinuity in the power train (sometimes referred to as a "negative gear-shifting overlap") is in general desirable, since it results in transient removal of load torque from the engine output shaft and consequent quick increase of the engine output shaft rotational speed, such speed increase of the engine output shaft being what should occur in a down-shifting operation. However, when resort is had to this expedient, care must be taken that the time duration of the discontinuity in the power train be maintained at a certain preselected value, so as to minimize the jerk or jolt that results with this particular down-shifting method. Again, with the known hydraulically operating control arrangement for controlling the engine transmission ratio, it is difficult and expensive, when possible at all, to adjust the time duration of this power-train discontinuity, for example to meet new operating conditions. Likewise, as with respect to the time duration of the "gear-shifting overlap," it is extremely difficult if not impossible to prevent viscosity fluctuations in the hydraulic oil from having a marked effect upon the time duration of the power-train discontinuity.